Multidrug and Toxin Extrusion (MATE) transporters are integral membrane proteins that move structurally unrelated lipophilic cations across the cell membrane by utilizing a preexisting sodium or proton gradient. Bacterial MATE transporters function as multidrug efflux pumps by expelling a cohort of antimicrobial agents from the cytoplasm, whereas their human counterparts mediate the excretion of various cytotoxic metabolites as well as therapeutic drugs. Given their functional relevance to the unwanted resistance to antimicrobials and chemotherapy, molecular structures of the MATE transporters will reveal not only how they transport their substrates across the cell membrane but also how their transport activity can be modulated in order to overcome drug resistance. We aim to elucidate the molecular structure of an intact MATE transporter using X-ray crystallography. To date we have obtained crystals that diffract better than 3.8 E-resolution. Based on the crystal structure, we will construct various MATE mutants, reconstitute them into liposomes and characterize their transport properties utilizing substrate uptake assays. Our long-term objective is to decipher the molecular basis for multidrug binding and transport. Specifically, we seek to (1) establish the structures of a MATE transporter with and without drug substrates; (2) probe the transport mechanism via functional reconstitution of purified MATE transporters in liposomes; (3) determine the structures and drug-binding specificities of various MATE mutants. Our work will provide new insights into the general principles that govern multidrug binding and transport; it will also set the stage for structure-based design of pharmaceuticals targeting drug-resistant human pathogens and cancer cells.